Hi,
My '99 Corolla just produced a check engine light, which turned out to be a P0446. I got it cleared. From reading on the web, it appears this is common on this model/year. It is appears to be the EVAP canister and/or solenoid. After a day it has not returned.
Now the question, is there a chance that is a one time event and will not return? Any experience?
It is possible, but not very likely that it was a one time event. I believe that trouble code has 2-trip detection logic, which means that the ECU has to see the same problem on 2 consecutive trips to turn on the light.
P0446 comes up when the system detects a leak in the vacuum switching valve (VSV) for the vapor pressure sensor, the charcoal canister, or the related lines or if there is an open or short in one of the system VSV's. This is one of those problems where you will probably need a factory repair manual to diagnose and repair. You can get access to the factory repair manuals for $10/day at techinfo.toyota.com.
P0446 will not be caused by a loose gas cap.
The vacuum switching valves during these years were junk. They stick and not do the ON/OFF thing the ECU commands them to do. However, there were batches of defective canisters during these years too, but they will set other codes with 0446.
Just keep an eye on it and Autozone can help read and clear the codes free. If it gets to be a problem, get a mechanically inclined friend to help clean the canister VSV first before throwing money at the problem. Just don't break the metal tabs or plastic caps on the VSV.
The components that can trigger P0446 are not directly connected to the gas tank, and if the gas cap were loose, the OP would get either an additional code or a different code. As a practical matter, the chances that a loose gas cap caused P0446 are pretty much zero. Free or not, I try not to give advice that has little or no chance of fixing the problem.
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